The invention relates to well bore explosive detonation tools and, in particular, to a hydraulic firing head for a downhole tool.
In subterranean well bores, firing heads are used to detonate downhole explosives. Explosives are used downhole in various tools including packers and perforating gun assemblies. In these tools, the firing head is driven to actuate an initiator to detonate explosive charges in the tool.
One type of firing head is driven hydraulically. These hydraulic firing heads are generally conveyed on a tubing string and controlled by fluid pressure applied through a fluid column in the tubing string. The fluid can be a liquid or gas for example, compressed nitrogen or water. Pressure is applied from surface through the fluid column in the tubing string above the firing head, acting on a piston and attached assembly, which is secured by a number of shear pins. The shear pins are selected and built to shear at a known load. The number of shear pins used to secure the piston determines the pressure at which the head fires. Shearing the pins by applied pressure, drives a firing pin attached to the piston to strike the initiator, which transfers an explosive charge to the detonator in association with the main explosives of the tool.
Once the explosives are detonated, it is sometimes useful to drain the hydraulic fluid from the tubing string, prior to tripping the tubing to surface. For this purpose, vents closed by sliding sleeves have been installed in the tubing string and in the firing head. However when vents have been included in the firing head, the pressures generated by evacuating fluid or gun detonation pressure tend to drive the piston to close the vents prior to complete draining of the tubing string. Snap rings have been used to lock the piston in a position away from vents. However, hydraulic firing heads often fail to allow complete draining of the tubing string.
A downhole tool hydraulic firing head has been invented that includes an automatic drain including vents, the opening of which is controlled by movement of the piston. A locking collet in the firing head holds the piston down after firing, ensuring that venturi action or gun detonation pressure acting on the piston does not close off the vents.
In accordance with a broad aspect of the present invention, there is provided a downhole tool hydraulic firing head comprising: a housing connectable into a tubing string and having a bore extending therethrough from its upper end to its lower end; a vent opening extending through a side wall of the housing, the vent opening being open to the bore; a piston in the bore, the piston being drivable by fluid pressure applied though the bore; a firing pin for activating detonation of an explosive charge, the firing pin connected to the piston to move with the piston; a sleeve on the piston and moveable therewith from a position covering the vent opening to a position clear of the vent opening; and a locking collet including a plurality of collet fingers with engaging lugs acting between the sleeve and the housing to lock the sleeve into the position where it is clear of the vent opening.
The housing can be formed of one part or multiple interconnected parts, as desired. Manufacture, assembly and repair can be facilitated by forming the housing of multiple interconnected parts. The piston, sleeve, firing pin can be formed integral with each other or of separate parts secured to move together. Again, the use of separate secured parts can facilitate manufacture, assembly and repair.
The sleeve can be the sidewall of the piston or a cylindrical extension of the piston. Preferably sealing means, such as O-rings are provided on the sleeve to seal against fluid passage through the vent opening when the sleeve is in position covering the vent opening.
The locking collet is disposed to act between the sleeve and the housing and engages a shoulder. The locking collet and shoulder can be disposed directly on these parts or can be disposed on other parts secured to the housing and the sleeve. The collet can be connected to move with the sleeve, while the shoulder is formed in the housing or, alternately, the collet can be secured to the housing, while the shoulder is in association with the sleeve.
In one embodiment, the locking collet is secured to the sleeve. Alternately, the locking collet can be secured to the piston or the firing pin, since both the sleeve and the firing pin move with the piston. Alternately the locking collet can be secured to another part, connected to at least one of the firing pin, the piston or the sleeve. Likewise, the shoulder can be formed directly on the housing or on parts secured within the housing.
The piston can be releasably secured against movement unless a selected amount of fluid pressure is applied to the piston. In one embodiment, a shear pin is used to releasably secure the piston in this way.